5 research outputs found
Universality of Symmetry and Mixed-symmetry Collective Nuclear States
The global correlation in the observed variation with mass number of the
and summed transition strengths is examined for rare earth nuclei. It is
shown that a theory of correlated and fermion pairs with a simple
pairing plus quadrupole interaction leads naturally to this universality. Thus
a unified and quantitative description emerges for low-lying quadrupole and
dipole strengths.Comment: In press, Phys. Rev. Lett. 199
Solution of the Nuclear Shell Model by Symmetry-Dictated Truncation
The dynamical symmetries of the Fermion Dynamical Symmetry Model are used as
a principle of truncation for the spherical shell model. Utilizing the usual
principle of energy-dictated truncation to select a valence space, and
symmetry-dictated truncation to select a collective subspace of that valence
space, we are able to reduce the full shell model space to one of manageable
dimensions with modern supercomputers, even for the heaviest nuclei. The
resulting shell model then consists of diagonalizing an effective Hamiltonian
within the restricted subspace. This theory is not confined to any symmetry
limits, and represents a full solution of the original shell model if the
appropriate effective interaction of the truncated space can be determined. As
a first step in constructing that interaction, we present an empirical
determination of its matrix elements for the collective subspace with no broken
pairs in a representative set of nuclei with . We demonstrate
that this effective interaction can be parameterized in terms of a few
quantities varying slowly with particle number, and is capable of describing a
broad range of low-energy observables for these nuclei. Finally we give a brief
discussion of extending these methods to include a single broken collective
pair.Comment: invited paper for J. Phys. G, 57 pages, Latex, 18 figures a macro are
available under request at [email protected]
Cold electrons at comet 67P/Churyumov-Gerasimenko
Context. The electron temperature of the plasma is one important aspect of the environment. Electrons created by photoionization or
impact ionization of atmospheric gas have energies �10 eV. In an active comet coma, the gas density is high enough for rapid cooling of
the electron gas to the neutral gas temperature (a few hundred kelvin). How cooling evolves in less active comets has not been studied
before.
Aims. We aim to investigate how electron cooling varied as comet 67P/Churyumov-Gerasimenko changed its activity by three orders
of magnitude during the Rosetta mission.
Methods. We used in situ data from the Rosetta plasma and neutral gas sensors. By combining Langmuir probe bias voltage sweeps
and mutual impedance probe measurements, we determined at which time cold electrons formed at least 25% of the total electron
density. We compared the results to what is expected from simple models of electron cooling, using the observed neutral gas density
as input.
Results. We demonstrate that the slope of the Langmuir probe sweep can be used as a proxy for the presence of cold electrons. We
show statistics of cold electron observations over the two-year mission period. We find cold electrons at lower activity than expected
by a simple model based on free radial expansion and continuous loss of electron energy. Cold electrons are seen mainly when the gas
density indicates that an exobase may have formed.
Conclusions. Collisional cooling of electrons following a radial outward path is not sufficient to explain the observations. We suggest
that the ambipolar electric field keeps electrons in the inner coma for a much longer time, giving them time to dissipate energy by
collisions with the neutrals. We conclude that better models are required to describe the plasma environment of comets. They need to
include at least two populations of electrons and the ambipolar field